This invention relates to a process for expanding the volume of tobacco and an apparatus for carrying out the process. More particularly, this invention relates to expanding tobacco using carbon dioxide.
The tobacco art has long recognized the desirability of expanding tobacco to increase the bulk or volume of tobacco. There have been various reasons for expanding tobacco. One of the early purposes for expanding tobacco involved making up the loss of weight caused by the tobacco curing process. Another purpose was to improve the smoking characteristics of particular tobacco components, such as tobacco stems. It has also been desired to increase the filling power of tobacco so that a smaller amount of tobacco would be required to produce a smoking product, such as a cigarette, which would have the same firmness and yet would deliver lower tar and nicotine than a comparable smoking product made of non-expanded tobacco having a more dense tobacco filler.
Various methods have been proposed for expanding tobacco, including the impregnation of tobacco with a gas under pressure and the subsequent release of pressure, whereby the gas causes expansion of the tobacco cells to increase the volume of the treated tobacco. Other methods which have been employed or suggested have included the treatment of tobacco with various liquids, such as water or relatively volatile organic or inorganic liquids, to impregnate the tobacco with the same, after which the liquids are driven off to expand the tobacco. Additional methods which have been suggested have included the treatment of tobacco with solid materials which, when heated, decompose to produce gases which serve to expand the tobacco. Other methods include the treatment of tobacco with gas-containing liquids, such as carbon dioxide-containing water, under pressure to incorporate the gas in the tobacco and when the impregnated tobacco is heated or the ambient pressure reduced the tobacco expands. Additional techniques have been developed for expanding tobacco which involve the treatment of tobacco with gases which react to form solid chemical reaction products within the tobacco, which solid reaction products may then decompose by heat to produce gases within the tobacco which cause expansion of tobacco upon their release. More specifically:
U.S. Pat. No. 1,789,435 describes a method and apparatus for expanding the volume of tobacco in order to make up the loss of volume caused in curing tobacco leaf. To accomplish this object, the cured and conditioned tobacco is contacted with a gas, which may be air, carbon dioxide or steam under pressure and the pressure is then relieved, the tobacco tends to expand. The patent states that the volume of the tobacco may, by that process, be increased to the extent of about 5-15%.
U.S. Pat. No. 3,771,533, commonly assigned herewith, involves a treatment of tobacco with carbon dioxide and ammonia gases, whereby the tobacco is saturated with these gases and ammonium carbamate is formed in situ. The ammonium carbamate is thereafter decomposed by heat to release the gases within the tobacco cells and to cause expansion of the tobacco.
U.S. Pat. No. 4,258,729, commonly assigned herewith, describes a method for expanding the volume of tobacco in which the tobacco is impregnated with gaseous carbon dioxide under conditions such that the carbon dioxide remains substantially in the gaseous state. Pre-cooling the tobacco prior to the impregnation step or cooling the tobacco bed by external means during impregnation is limited to avoid condensing the carbon dioxide to any significant degree.
U.S. Pat. No. 4,235,250, commonly assigned herewith, describes a method for expanding the volume of tobacco in which the tobacco is impregnated with gaseous carbon dioxide under conditions such that the carbon dioxide remains substantially in the gaseous state. During depressurization some of the carbon dioxide is converted to a partially condensed state within the tobacco. That patent teaches that the carbon dioxide enthalpy is controlled in such a manner to minimize carbon dioxide condensation.
U.S. Pat. No. Re. 32,013, commonly assigned herewith, describes a method and apparatus for expanding the volume of tobacco in which the tobacco is impregnated with liquid carbon dioxide, converting the liquid carbon dioxide to solid carbon dioxide in situ, and then causing the solid carbon dioxide to vaporize and expand the tobacco.
Copending and commonly-assigned U.S. patent application Ser. No. 07/717,064, filed Jun. 18, 1991, discloses a process for impregnating tobacco with carbon dioxide and then expanding the tobacco. That disclosed process includes steps of contacting tobacco with gaseous carbon dioxide and controlling process conditions to cause a controlled amount of carbon dioxide to condense on the tobacco.
It has been found that with gaseous carbon dioxide impregnation processes, the tobacco must achieve a sufficiently low exit temperature at the end of the process (after the venting of carbon dioxide from maximum pressure) in order for the tobacco to be successfully impregnated. During venting, the escaping carbon dioxide lowers the temperature of the tobacco bed.
Prior processes for impregnating tobacco using gaseous carbon dioxide without controlled condensation cannot achieve sufficient cooling of a high bulk density tobacco bed because cooling is provided only by gas expansion. As the bulk density of the tobacco bed increases, the mass of tobacco to be cooled increases and the volume or void space remaining within the tobacco bed and the available gas for cooling decreases. Without sufficient cooling, an acceptable pre-expansion stability of the impregnated tobacco cannot be achieved.
Typically, a loosely filled tobacco bed exhibits a tobacco bulk density gradient with a higher bulk density toward the bottom due to the compressing effect of the weight of the column of tobacco. Tobacco expansion processes using gaseous carbon dioxide and loosely filled tobacco beds of relatively low bulk density may result in non-uniform cooling of the tobacco and thus non-uniform stability and expansion of the tobacco.
The bulk density at the bottom of a deep tobacco bed may be the limiting factor in a gas-only process, because the tobacco at the bottom of a deep bed may have too great a bulk density to be efficiently cooled by gas expansion cooling. As a result, tobacco expansion processes using gaseous carbon dioxide are limited to relatively small or shallow tobacco beds. While such small beds have been used for experimental development, they were not usually commercially practical.